Wavelength conversion

ABSTRACT

A wavelength conversion device comprising a plurality of regions. Each region for absorbing radiant energy and emitting light having a characteristic dependent upon which region of the device emits the light.

PRIORITY

This applicant claims priority of provisional patent application No.61/308,626 filed 26 Feb. 2010.

FIELD OF THE INVENTION

This invention relates to the field of optical components, moreparticularly to optical components that convert energy from onewavelength to another, more particularly optical components that convertenergy from one wavelength to a visible wavelength, and display systemsincorporating such wavelength conversion components.

BACKGROUND OF THE INVENTION

A variety of useful devices receive radiant energy in a first wavelengthband and emit radiant energy in a different wavelength band. Forexample, periodically poled crystals, such as lithium niobate, are usedto double the frequency of laser illumination and phosphors aredeposited on the inside of cathode ray tubes to convert a stream ofelectrons into visible light.

One such wavelength converter is a phosphor color wheel that receivesradiant energy and emits visible light. The phosphor color wheel is usedto enable a solid state illuminator, such as a laser emitter, to replacethe high pressure arc lamps traditionally used in projection displays.Solid state illuminators, such as light emitting diodes (LEDs) and laseremitters, have the potential to provide a very long lifetime and widergamut compared to arc lamps.

Solid state illuminators, however, have drawbacks. The luminance,available flux within the etendue of a spatial light modulator (SLM),provided by an LED is limited, and high-power LEDs are difficult to coolin small packages. Laser sources have a very high luminance, but tend tocreate image speckle artifacts due to their extremely narrow bandwidth.Speckle artifacts are difficult and costly to eliminate in frontprojection display systems.

What is needed is low cost, efficient method of converting radiantenergy from one wavelength band to another wavelength band in order toprovide a long-life high-luminance illumination source that does notintroduce any additional artifacts into the light beam.

SUMMARY OF THE INVENTION

Objects and advantages will be obvious, and will in part appearhereinafter and will be accomplished by the present invention whichprovides a method and system for wavelength conversion.

One embodiment of the claimed invention provides a method of convertingradiant energy. The method comprises: directing a first radiant energybeam at a moving surface; absorbing a portion of the first radiantenergy beam; emitting a second radiant energy beam, the second radiantenergy beam having a characteristic dependent upon the composition ofthe moving surface, wherein different regions of the moving surface emitradiant energy having different characteristics; and moving the movingsurface relative to the first radiant energy band such that the firstradiant energy band impinges on different portions of the movingsurface, wherein the regions are arranged in annular bands on the movingsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of a phosphor color wheel of the prior art.

FIG. 2 is a schematic view of a illumination system using the colorwheel of FIG. 1.

FIG. 3 is a schematic view of an illumination system according to oneembodiment of the present invention.

FIG. 4 is a schematic view of an illumination system according toanother embodiment of the present invention.

FIG. 5 is a schematic view of an illumination system according toanother embodiment of the present invention.

FIG. 6 is a schematic view of an illumination system according toanother embodiment of the present invention.

FIG. 7 is a schematic view of an illumination system according toanother embodiment of the present invention.

FIG. 8 is a schematic view of an illumination system according toanother embodiment of the present invention.

FIG. 9 is a schematic view of a projection display system utilizing animproved illumination system according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plan view of a typical phosphor-based color wheel 100 of theprior art. The wheel, is comprised of multiple color segments 102, 104,106, each of which is used to generate light having a particular coloras will be explained later. The color segments 102, 104, 106 areattached to a central hub 108 which has a central aperture 110 throughwhich a shaft is attached to the color wheel 100.

While three segments are shown in FIG. 1, the color wheel 100 of FIG. 1may include more segments. For example, many color wheels include asingle segment for each of three primary colors such as red, green, andblue. Other color wheels include multiple segments for a singlecolor—either adjacent to segments of the same color or separated bysegments of another color. Color wheels may also include additionalcolors such as cyan, magenta, and yellow, and may provide segmentsintended to generate white light—often called white or clear segments.While three primary colors have traditionally been used in displaysystems, the term multi-primary is typically reserved for systemsutilizing four or more primary colors.

FIG. 2 illustrates the use of a typical phosphor-based color wheel 100in an illumination system 200 of the prior art. The color wheel ismounted on a motor 202 by a shaft, or coupled to a motor by a belt,gear, or some other coupler to enable the motor 202 to turn the colorwheel 100. The color wheel 100 is positioned in the path of a light beam204 generated by a light source 206 and focused on the color wheel by anoptical system 208. Light source 206 may be any source of radiantenergy, and typically is one or more LEDs or lasers. While shown as asingle lens, optical system 208 often is a lens system comprised ofmultiple lenses.

Light focused on the phosphor-based color wheel 100 at point 210 excitesphosphors in or on the color wheel. As described in U.S. Pat. No.7,547,114 B2, issued 16 Jun. 2009 to Li et al. and entitled MulticolorIllumination Device Using Moving Plate With Wavelength ConversionMaterials, U.S. Pat. No. 7,726,861 B2, issued 1 Jun. 2010 to Xu andentitled Brightness Enhancement With Directional Wavelength Conversion,and U.S. Pat. No. 7,744,241 B2, issued 29 Jun. 2010 to Xu and entitledHigh Brightness Light Source Using Light Emitting Devices Of DifferentWavelengths And Wavelength Conversion, various phosphors may be appliedon a substrate to enable a conversion from one wavelength band toanother wavelength band. While this disclosure will discuss thephosphors as being applied to the substrate or located on the substrate,it is understood that this includes phosphors that are embedded in thesubstrate or applied to a back surface of the substrate or sandwichedbetween layers of the substrate.

The phosphor color wheel absorbs energy from the illumination beam 204and reemits the beam 212 on the other side of the wheel. The emittedbeam 212 may be collected and focused or collimated by optics 214.

The illumination beam 204 and the emitted beam 212 have differentwavelengths at least a portion of the time. For example, segment 102 mayinclude red phosphors —or phosphors that emit light in what is perceivedby humans as red light —while segments 104 and 106 include green andblue phosphors respectively. If laser 206 emits blue light 204, when thered segment 102 is in the light path the red phosphors will absorb theblue laser light and reemit red light 212. When the green segment 104 isin the light path the green phosphors will absorb the blue laser lightand reemit green light 212. When the blue segment 106 is in the lightpath the blue phosphors will absorb the blue laser light and reemit bluelight 212.

While the illumination system of FIG. 2 provides an efficient method andsystem for generating light of various colors, when used in a displaysystem it has serious disadvantages. For example, while the use of solidstate light sources such as LEDs or lasers provides an efficient sourceof radiant energy to stimulate the phosphors, the use of a segmentedcolor wheel constrains the display system to the use of single colordisplay periods that are determined by the relative sizes of the variouscolor segments 102, 104, 106. As such, no matter the color needs of theimage being generated, a display system using a segmented color wheel100 can only devote a portion of time to a particular color that isdetermined by the size of a particular color filter relative to thesizes of the remaining color filters as it is generally necessary torotate the wheel at a constant velocity.

The new color wheel design and illumination system 300 shown in FIG. 3overcomes this limitation. The color wheel 302 of FIG. 3 includesmultiple tracks for bands 304, 306, 308 around the color wheel ratherthan radial segments. These tracks enable the beam of radiant energy 310produced by source 312 to stimulate the phosphors on any given track forany proportion of the time. This enables the improved color wheel 302 tobe used in illumination systems 300 that provide primary color periodshaving durations that vary relative to one another, generally based onthe color needs of an image being generated or on the illuminationcapabilities of the source 312 and phosphors used to convert the sourceradiant energy 310. For example, a bluish image may increase therelative time the radiant energy 310 lingers on a blue phosphor bandcompared to a yellow or red band.

Because the illumination system of FIG. 3 does not include radialspokes, a mechanism for moving the beam of radiant energy 310 ontodifferent segments of the color wheel 302 is used. This mechanism mayinclude a servo or other mechanism to move the color wheel itselfrelative to the radiant energy beam 310, or a mechanism to move the beamrelative to the color wheel. As shown in FIG. 3, a tilting mirror 314may be used to direct the radiant energy 310 from one track to anotheron the color wheel.

In systems that move the beam relative to a stationary spinning colorwheel, the light beam reemitted by the wheel also moves relative to thewheel. In the example shown in FIG. 3, an optional integrator rod 316 isused to collect the light from all three of the phosphor tracks. Theintegrator rod collects the reemitted light and emits the reemittedlight from a far end of the integrator light. Most of the light passingthrough the integrator rod reflects from the integrator rod severaltimes as it passes through the integrator rod such that light passingthrough the exit of the integrator rod 316 is homogenized.

A controller, not shown, activates the tilting mirror 314 to selectwhich of the phosphor tracks is impinged by the radiant energy 310 fromthe source 312. The controller may sequentially alternate between all ofthe tracks, may select only a single track, or may alternate between asubset of the tracks. The duration each track is illuminated may beequal or unequal compared to the duration other tracks are illuminated.The controller may also alter the intensity of the radiant energy 310produced by the source 312 during or between the illumination periodsfor each track depending on the intensity needs of the illuminationsystem.

In the system of FIG. 3 and the following figures and embodiments, thesource 312 may produce a visible or invisible beam of radiant energy310. For example, the beam of radiant energy 312 may be ultravioletlight, infrared light, visible light, microwave energy, a beam ofelectrons, or any other suitable beam of radiant energy.

In the system of FIG. 3 and the following figures and embodiments, thecolor wheel may be wheel shaped, or may have other shapes. For example,the color wheel may be drum shaped or formed on a belt. The term colorwheel and the illustration of the color wheel are selected only becausea disc-shaped color wheel is the most popular embodiment used incontemporary filter color wheel based display systems and those skilledin the art are familiar with existing color wheels.

In the system of FIG. 3 and the following figures and embodiments, thecolor wheels illustrated as transmissive color wheels may be reflectiveinstead, and vice-versa. The addition of a reflective surface to a farside of a color wheel may be used to convert a transmissive color wheelinto a reflective color wheel. The reflective surface may reflect all oronly some of the wavelengths of interest in the illumination system. Forexample, if a blue laser source is used, it may be desired to allow theblue light to pass through the wheel while reflecting light from some orall of the other portions of the visible light spectrum. The reflectivesurface may be a portion of the moving color wheel—in which case thewheel truly is reflective, or may be an independent or stationaryreflector positioned by the color wheel.

In the system of FIG. 3 and the following figures and embodiments, thecolor wheel is illustrated and discussed as having multiple tracks ofdifferent colors. It should be understood that one or more of the tracksmay be devoid of significant phosphors such that the radiant energy usedto illuminate the track is not significantly converted by the phosphors.For example, if the source 312 produces radiant energy 310 in awavelength band that is useful for the illumination system, one or moretracks of the color wheel may be clear in order to allow the radiantenergy to pass through the color wheel without conversion.

In the system of FIG. 3 and the following figures and embodiments,optics 318 is illustrated as a single refractive lens. It should beunderstood that the illustration of the optics 318 or other opticalcomponents as a single refractive lens is to simplify the illustrationand is not a limitation of the system unless otherwise stated. Optics318 may be comprised of more than one optical element and each opticalelement may be refractive, diffractive, reflective, or any other type ofoptical element, with or without optical power.

In the system of FIG. 3 and the following figures and embodiments, theorder or placement of the various optical components is illustratedschematically for purposes of illustration only and should not beconsidered as limiting. For example, the relative locations of thetilting mirror 314 and optics 318 may be interchanged or the tiltingmirror 314 may be placed between individual elements of the optics 318.Likewise, the integrator rod may be placed before the color wheel orafter the color wheel. As the phosphors are generally dispersive, theintegrating rod may not be used in some embodiments. An integrating rodis useful, however, to collect light as emitted from various tracks ofthe color wheel. As the reemission of the energy from the phosphor wheeltakes a finite amount of time, the color wheel emits light from anelongated area of the color wheel. The shape and location of theelongated strip depends on the speed of the color wheel, the decayperiod of the phosphors, and the energy level used to excite thephosphors.

The tilting mirror 314 of FIG. 3 may be a gimbaled mirror or an array ofgimbaled mirrors. The tilting mirror may also be a single tilting mirrorsuch a scanning mirror used in scanning displays and printers, or anarray of such scanning mirrors.

FIG. 4 illustrates an illumination system 400 according to anotherembodiment of the present invention. In FIG. 4, radiant energy 310 fromthe source 310 is once again deflected by the tilting mirror 314 throughoptics 318 to an integrating rod 416. In this embodiment, however, aninput face of the integrating rod 416 includes multiple regions 418,420, 422 on which various phosphors have been deposited. Each region onthe input face of the integrating rod performs a similar function to thefunction of the color wheel in FIG. 3—the conversion of the inputradiant energy 310 to a beam of energy in a desired wavelength or thepassing of the radiant energy 310.

As shown in FIG. 4, the tilting mirror 314 may be used to direct theradiant energy 310 to each individual region of the integrating rod. Forexample, path 424 illustrates the radiant energy being applied to acenter region 420 on the input face of the integrating rod 416 whilepath 426 illustrates the radiant energy being applied to another region418 of the input face.

In additional to the motion shown in FIG. 4 that moves the source beamof radiant energy from one region of the input face of the integratingrod to another region of the input face of the integrating rod, it maybe desired to move the point at which the radiant energy 310 impinges onthe input face in order to prevent excessive localized heating of thephosphors or the input face or coatings on the input face. It isexpected that radiant energy 310 impinging on the input face will have agreat enough power level to damage or destroy the phosphor coatings onthe input face of the integrating rod 416. Embodiments using an actualcolor wheel generally spin the color wheel fast enough to avoid thelocalized heating that can destroy the coatings. Embodiments using colorwheels may introduce additional motion of the input beam, generally in adirection perpendicular to the rotational direction relative to the beamat the point where the beam illuminates the wheel. The motion across aparticular region of the input face of the integrator rod may be linear,circular, elliptical, random, vibratory, or follow a Lissajous or anyother pattern. The rotation speed of the color wheel and the movement ofthe beam across a particular region are for purposes of cooling,providing uniformity, and to prevent damage to the coatings. Thus, therotation speed or movement within a given track or region is independentto the frame rate of a display system using the illumination system.This allows the wheel or tilt mirror to operate at speeds much lower aswell as much faster than the speeds required by a traditional colorwheel and allows the noise generated by the movement to not only bereduced, but also to be generated at frequencies outside the hearing ofa human—which can result in significantly quieter operation.

FIG. 5 is a schematic view of an illumination system according toanother embodiment of the present invention. In FIG. 5, multiple sources512 are used to generate multiple source beams of radian energy. Themultiple sources may each produce a beam of radiant energy in the samewavelength or band of wavelengths, or they may produce beams of radiantenergy in different bands. The multiple source may operatesimultaneously or sequentially or any combination thereof The beam ofradiant energy from each of the multiple source or groups of themultiple sources or any combination thereof may be directed towardindividual tracks on the phosphor based color wheel 302 such thatactivation of a particular source or sources stimulates the phosphors ofa different color.

The use of multiple source directed to the multiple tracks or regions onthe color conversion device allows the elimination of a tilting mirroror other mechanism to direct the radiant energy to a particular track.

Embodiments utilizing multiple sources 512 to generate additional powermay utilize a tilting mirror as shown in other embodiments. The tiltingmirror shown in other embodiments may also be replaced, in theembodiment of FIG. 5, the embodiments shown in FIGS. 3 and 4, and otherembodiments, by an acousto-optical modulator, Bragg filter, switchableBragg grating, holographic modulator, or other device as illustrated bymodulator 514.

Modulator 514 may be controlled to direct the radian energy from one ormore of the sources 512 to one or more of the tracks on the color wheel302.

FIG. 6 is a schematic view of yet another embodiment of the presentinvention illustrating the use of a color wheel having tracks instead ofradial segments. In FIG. 6, the modulator 602 directs the source radiantenergy along one or more multiple paths to reflectors 604 and 606 whichdirect the radiant energy to the color wheel 302. Modulator 602 may be amodulator as described in FIG. 5, or it may be a scanning mirror, a tiltmirror, or an array of mirrors such as a micromirror device. A digitalmicromirror device may be used in this embodiment in a bistable mannerto direct light to one or the other of the reflectors 604 and 606.Separate regions, or interleaved regions, of the micromirror array mayalso be activated in opposite directions to direct different portions ofthe incident radiant energy to one or the other or both of thereflectors.

While FIG. 6 illustrates the use of two separate reflectors, bothlocated in the plane of the drawing, it should be understood that thereflectors may be regions of the same reflector, may be situated on thesame side of the path of incident radiant energy, there may be more thantwo reflectors, and the reflectors maybe positioned in any position inthree-dimensions.

The modulators and reflectors shown in FIG. 6 and the variousembodiments described herein may include coatings designed to modify orlimit the band of radiant energy. Likewise, the color wheel or theintegrating rod may also have coatings to modify or limit the band ofradiant energy allowed to pass through the illumination system.

FIG. 7 is a schematic view of one embodiment of the illumination system700 using a reflective color wheel 702. Radiant energy from theillumination sources impinges on the reflective color wheel 702 where itis absorbed and reemitted as radiant energy in another band ofwavelengths. The incident illumination is shown by beams from thesources while the reflected beam is shown as a dispersed beam. Adichroic splitter 704 is used to reflect one of the incident andreflected beams while passing the other of the incident and reflectedbeams. In FIG. 7, the incident beam is passed through the dichroicsplitter while the beam reemitted by the reflective wheel is reflectedby the dichroic splitter. FIG. 8 shows the system of FIG. 7 at anotherpoint in time when the beams are directed to another track of the colorwheel 702. The beams may be directed between the tracks by any of themethods discussed above.

FIG. 9 is a schematic view of a display system 900 utilizing theillumination systems 902 described above. Light from the illuminationsystem 902 is directed to a transmissive or reflective spatial lightmodulator 904, such as a liquid crystal on silicon or micromirrormodulator. The modulator and illumination system are controlled bycontroller 908 which also received image data describing a desired imageto be produced. The spatial light modulator 904 modulates the incomingbeam of light from the illumination system to form an image on imageplane 906. Some spatial light modulators absorb the light not used toform the image, some spatial light modulators transmit the unused lightin a different direction to a different location such as the opticaldump shown in FIG. 9.

Thus, although there has been disclosed to this point a particularembodiment for a wavelength converter and method therefore, it is notintended that such specific references be considered as limitations uponthe scope of this invention except insofar as set forth in the followingclaims. Furthermore, having described the invention in connection withcertain specific embodiments thereof, it is to be understood thatfurther modifications may now suggest themselves to those skilled in theart, it is intended to cover all such modifications as fall within thescope of the appended claims. In the following claims, only elementsdenoted by the words “means for” are intended to be interpreted as meansplus function claims under 35 U.S.C. §112, paragraph six.

What is claimed is:
 1. A method of converting radiant energy comprising:directing a first radiant energy beam at a moving surface; absorbing aportion of the first radiant energy beam; emitting a second radiantenergy beam, the second radiant energy beam having a characteristicdependent upon the composition of the moving surface, wherein differentregions of the moving surface emit radiant energy having differentcharacteristics; and moving the moving surface relative to the firstradiant energy beam such that the first radiant energy beam impinges ondifferent portions of the moving surface, wherein the regions arearranged in annular bands on the moving surface.
 2. The method of claim1, wherein different phosphors are applied to the different regions, andthe second radiant energy beam has a characteristic dependent upon thecomposition of the different phosphors.
 3. The method of claim 2,wherein the regions are concentric rings of different color wavelengthemitting phosphors.
 4. The method of claim 3, wherein the first radiantenergy beam comprises a beam of laser light.
 5. The method of claim 4,wherein the beam of laser light is a beam of blue laser light.
 6. Themethod of claim 5, wherein the regions comprise a first ring of redlight emitting phosphors and a second ring of green light emittingphosphors.
 7. The method of claim 6, wherein the regions furthercomprise a ring of blue light emitting phosphors.
 8. The method of claim7, wherein moving the moving surface relative to the first radiantenergy beam comprises activitating a tilting mirror to select which ofthe phosphor rings is impinged by the blue laser light.
 9. The method ofclaim 7, wherein moving the moving surface relative to the first radiantenergy beam comprises selectively energizing ones of multiple sources ofbeams respectively directed toward individual ones of the rings.
 10. Themethod of claim 7, wherein moving the moving surface relative to thefirst radiant energy beam comprises directing the blue laser light usinga digital micromirror device.
 11. The method of claim 1, wherein theregions are concentric rings of different color wavelength emittingphosphors.
 12. The method of claim 1, wherein the first radiant energybeam comprises a beam of laser light.
 13. The method of claim 1, whereinthe regions comprise a first ring of red light emitting phosphors and asecond ring of green light emitting phosphors.
 14. The method of claim1, wherein moving the moving surface relative to the first radiantenergy beam comprises activitating a tilting mirror to select which ofthe phosphor rings is impinged by the blue laser light.
 15. The methodof claim 1, wherein moving the moving surface relative to the firstradiant energy beam comprises selectively energizing ones of multiplesources of beams respectively directed toward individual ones of therings.
 16. The method of claim 1, wherein moving the moving surfacerelative to the first radiant energy beam comprises directing the bluelaser light using a digital micromirror device.
 17. The method of claim1, wherein the moving surface comprises a color wheel comprising arotable disc, the regions are regions of photoluminescence materialapplied to the rotatable disc, and the second radiant energy beam isemitted by the photoluminescence material.
 18. The method of claim 17,wherein the first radiant energy beam comprises a beam of blue laserlight, and the wheel is configured relative to a reflective surface toallow blue light to pass through the while reflecting light from atleast some of other color portions of the visible light spectrum. 19.The method of claim 17, wherein the color wheel is configured fortransmitting at least a portion of the second radiant energy beamemitted by the photoluminescence material.
 20. The method of claim 17,wherein the color wheel is configured for reflecting at least a portionof the second radiant energy beam emitted by the photoluminescencematerial.